This multicenter trial tests whether prophylactic magnesium sulfate given to women, for whom preterm delivery is imminent, reduces the risk of death or moderate to severe cerebral palsy in their children. Women presenting from 24.0 to 31.6 weeks gestation with advanced preterm labor or premature rupture of the membranes (pPROM) and no recent exposure to magnesium sulfate are randomized to receive either intravenous magnesium sulfate or masked study drug placebo. The study drug is administered as a 6 gram loading dose followed by a 2 gram/hour infusion (or equivalent rate for placebo). If after 12 hours, delivery has not occurred and is not anticipated, the infusion is stopped. No other parenteral tocolytics other than the IV medication may be used. Retreatment with study medication is given any time labor recurs or delivery is anticipated until gestational age is > 34.0 wks. Standard clinical management and therapy is to be maintained for all study patients. Patients are assessed for signs of intolerance to the study medications and maternal data are collected up to hospital discharge. A sample of venous blood is collected and neonatal cranial ultrasounds are performed. Up to three follow-up visits are scheduled over two years where certified examiners, masked to study group assignment, collect physical and neurological data, including a modified Gross Motor Function Classification Scale. The Bayley Scales of Infant Development is also administered. Cranial ultrasounds are reviewed centrally.